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Reactive Loading on LM4780 (but also general audio amplifier applications)

Mattia Benato
Prodigy 150 points

Hi,

I have a question about the "typical" circuit that you can see in different datasheets of audio amplifier IC.

The circuit is the parallel beetween a 10 Ohm resistor and a 0.7 uH inductor that you can found at the output of an audio amplifier.

How can I choose them respecting power considerations?

In the datasheet of LM4780 there aren't any considerations about this argument and also in other datasheets..

I thought to choose an inductor that support the Ipeak of the audio amplifier and for the resistor, a resistor that allow right power dissipation calculated by Vrms*Irms, but how can I calculate or estimate Vrms and Irms?

Thank you for help, I'll be very grateful to you if anyone attach a document about this argument.

Thanks

  • 0
    TI__Guru 52115 points
    Hi Mattia,

    I will try to answer your question tomorrow.

    Andy
  • 0
    TI__Guru 52115 points
    Hi Mattia,

    Here are some comments for you:

    "As you know well, in general amplifier may have oscillation issue with capacitive load. For this LM4780 load is speaker and sometime customer takes long speaker cable. In such a case, the amplifier will have capacitive load and may have oscillation problem.

    The inductance on the output is to isolate the cap load from amp output in high freq region. Then the amplifier has no OSC. The 10 ohms is to form the inductor over the resistance. If customer takes air core type inductor, the 10 ohms resistor is not necessary. So customer needs to consider flowing current capability. And the power consumption is almost none. Maybe ESR of the inductor is very low like 10m ohm or less. If customer make the inductor over the resistor, the resistor doesn’t have current flow. Because almost all current runs on the inductor. So the resistor value is not important."

    Andy
  • 0 in reply to Andy Liu (BMS)
    Prodigy 150 points

    Hi Andy, Thank you for your answer.

    I agree with you, in fact looking better the problem I think that an inductor of 7uH, for example, should work fine, infact it present an ideal impedance of 880 uOhm at 20 Hz (without considering ESR) and an 880 mOhm at 20 kHz that has a low contribute at this frequncy but it became 8.8 Ohm at 200 kHz reducing output power by a factor of 2 for all signals at this frequency.However the inductor must support all the current request from load(the speaker).

    But I don't understand why a 10 Ohm resistor is used...what do you mean with "10 ohms is to form the inductor over the resistance"?

    Thank you

    Mattia

  • 0 in reply to Mattia Benato
    TI__Guru 52115 points
    Hi Mattia,

    You can find the explanations about reactive loading in Page 22 of the datasheet. The 10 Ohm works to reduce the Q of the resonant performance.

    Andy